Metal nanoparticles and nanowires are the subject of current research efforts motivated by their high potential utility derived from nanoscale induced optical, electrical and chemical properties.
A wide range of techniques has been reported to synthesize metal nanoparticles including numerous high vacuum approaches as well as a range of photochemical [1-3] and thermal methods [4-7]. A technique that is just beginning to gain attention is the potential use of zeolite surfaces to induce the growth of metal nanostructures [8-10]. With many of their properties manifested on a nano- and subnano-dimensional scale, molecular sieves would appear to be excellent candidates to be in the vanguard of such nanofabrication efforts [11].
Unfortunately, current techniques for the generation of metal nanoparticles, such as nanosilver, are expensive and cumbersome [14]. Sub-nanometer silver ensembles can be induced to form within zeolite cavities under certain conditions [15-18], and much larger configurations often form on zeolite surfaces under reductive atmospheres. While metals readily congregate on zeolite surfaces, achieving stable, zeolite supported metal nanoscale structures has proved difficult because of the high metal mobility generally seen on zeolite surfaces. Typically, upon reduction, metals ion-exchanged into zeolite crystals diffuse to the crystal surface and rapidly coalesce into micron-scale agglomerates [19, 20]. Because of the low surface to volume ratio of these agglomerates (compared to nanometal ensembles), they generally behave like bulk metals, not displaying the novel properties anticipated for nanoparticulates.
Nanoparticulate silver has many potential uses. Many useful properties might be expected if inexpensive nanostructured silver materials were readily available. Silver is a well-known antimicrobial agent and nanoscale silver is finding increasing usage in medical devices, bandages and related medical applications [12, 13]. Current methods to generate nanosilver center on complex techniques such as surface sputtering. Research level work in biomedical engineering implants is showing promise in nanosilver bone cements where nanoparticle size control ranges from 5 nm to 50 nm [21].
Powerful surface plasmon absorption of nanoparticulate silver makes them particularly useful in applications such as biosensors, for example. Silver nanodots may be photo-fluorescence markers, which make them useful for a number of medical and similar applications. They are environmentally and biologically benign. Other exemplary silver nanodot applications include smart windows, rewritable electronic paper, electronic panel displays, memory components, and others.
A wide range of techniques has been reported to synthesize metal nanodots. Silver nanodots and their formation have recently been discussed by Metraux and Mirkin, 2005 [14]. Traditional methods for the production of silver nanodots require use of potentially harmful chemicals such as hydrazine, sodium borohydride and dimethyl formamide (“DMF”). These chemicals pose handling, storage, and transportation risks that add substantial cost and difficulty to the production of silver nanodots. A highly trained production workforce is required, along with costly production facilities outfitted for use with these potentially harmful chemicals.
Another disadvantage of known methods for producing silver nanodots relates to the time and heat required for their production. Known methods of production utilize generally slow kinetics, with the result that reactions take a long period of time. The length of time required may be shortened by some amount by applying heat, but this adds energy costs, equipment needs, and otherwise complicates the process. Known methods generally require reaction for 20 or more hours at elevated temperatures of 60° to 80° C., for example. The relatively slow kinetics of known reactions also results in an undesirably large particle size distribution and relatively low conversion. The multiple stages of production, long reaction times at elevated temperatures, relatively low conversion, and high particle size distribution of known methods make them costly and cumbersome, particularly when practiced on a commercial scale.
While silver ensembles are well known to form within zeolite cavities under certain conditions, and much larger configurations often form freely on zeolite surfaces, nanodots have not been known to form on zeolite surfaces in concentrations higher than trace levels.
These and other problems with presently known methods for making silver nanodots are exacerbated by the relatively unstable nature of the nanodots. Using presently known methods, silver nanodots produced have only a short shelf life since they tend to quickly agglomerate.
Xenon is present in ambient air at a concentration of 0.087 ppm and is currently derived from air by distillation. Companies specializing in air separation have developed techniques for xenon extraction from air [36-38]. Currently, most of the xenon produced in the world is used in specialized lighting. Other applications include nuclear medicine and laser applications. If it was economical to use, xenon might find widespread application as an anesthetic, having been referred to as ideal [35]. Characteristics of the xenon market and its applications have been reviewed and summarized by Hammarland [39].
Presently, a mixture of krypton and xenon is obtained from an oxygen stream in air distillation. The krypton and xenon are then further separated by cryogenic methods. Due to the high energy requirements of this cryogenic recovery, several alternative processes have been proposed. Certain polymer membranes have shown promise for the separation of xenon from air [40]. Efficient xenon selective adsorbents might allow not only more economical xenon capture from the atmosphere but could conceivably be employed to recapture and recycle xenon from an operating room environment, dramatically cutting its cost per use.
Although the strength of the interaction between silver zeolites and noble gases decreases markedly in the order Xe>Kr>Ar, the sorption affinity for argon is still significant, and some silver zeolites possess the unique property of being measurably selective in adsorbing argon over oxygen. silver mordenite has been reported to manifest at least some argon selectivity (vs. oxygen) [49]. Pressure swing adsorption simulations and experiments were successfully performed for the purification of oxygen (to at least 99.7% purity) from a feed gas comprising of 95% O2 and 5% Ar at 60-90° C. [49]. While silver mordenite appears to be the most widely reported zeolite-based argon selective adsorbent, silver exchanged zeolite X [50], silver exchanged Li—Na-LSX zeolite [51,52], silver exchanged zeolite A [53,54], Y, L, BEA, and ZSM-15 [28] have all been reported to show some degree of argon selectivity (vs. oxygen).
Existing methods for the separation of oxygen and argon are based upon adsorbents that show selectivity for oxygen over argon. However, the separation of argon and oxygen by adsorption-based methods is difficult due to the similar diameter and polarizability of the Ar atoms and O2 molecules. However, molecular sieves and microporous polymers with some degree of selectivity for oxygen are known and have been applied since the 1960s for the chromatographic resolution of Ar, O2, and N2 and other analytical purposes [41-46]. Oxygen (over argon) kinetic selectivity in certain carbon adsorbents has been employed for the production of purified oxygen and argon by pressure swing adsorption (PSA) [47,48].
Nitrogen also interacts strongly with silver exchanged zeolites. The nitrogen adsorption capacity and isosteric heat of adsorption of fully exchanged zeolite Ag—X was found to be significantly higher than that of Na—X and Li—X [55]. This effect was explained by means of the π-complexation mechanism, which would involve donation of the π-bond electrons of the N2 molecule to the empty s orbital of Ag+, and back-donation of electrons from the d orbital of silver to the empty π-antibonding orbital of N2 [55]. The basic concept for π-complexation was first described by Dewar [56]. The N2/O2 selectivity of Ag—X zeolite is also reported to be higher than for other cations. This effect has also been explained according to the π-complexation theory. The π-antibonding electrons of the O2 molecule do not allow the back-donation of electrons from the silver d orbitals. The bonding strength of N2 is too strong for practical PSA separations. However, it has been reported that combining the potentials of lithium and silver in hybrid Li—Ag—X zeolite can be superior to Li—X for air separation under certain circumstances. It has also been reported that a small amount of substitution of Ag in Li—X can improve N2/O2 separation properties [57]. Other silver exchanged zeolites, such as mordenite [58] and zeolite A [54], have been reported to have enhanced N2 capacities and N2/O2 selectivities compared to materials without silver.
There is also a need in the art for improved adsorbents for noble gases, xenon and argon in particular, which mitigates the disadvantages of the prior art, and there is a need in the art for methods of producing such improved adsorbents.